This paper reports on a time-accurate simulation of discrete tip injection upstream of a transonic axial compressor, NASA Rotor-67. Twenty two discrete injectors were placed around the casing upstream of the blade to investigate the effect of injector-rotor interaction at near-stall condition. Time-accurate simulations were performed with and without tip injection at stable and unstable operating points. Although the injected mass flow rate is very small, the range extension obtained promises the effective use of such injection in suppressing rotating stall at early stages of formation, with almost no efficiency penalty incurred. The effect of injection on the tip flow structure and unsteady response of the leakage flow are presented and discussed. Results indicate that injection periodically pushes the tip leakage vortex and passage shock rearward. The location of the leakage vortex with injection was found to be backward of that without injection, at the near-stall condition of the non-injection case. At the near-stall point of the injection case, however, the tip leakage vortex was at some situations rearward and at some situations forward, as compared with the non-injection case. In other words, the leakage vortex in the injection case oscillates around the location of the time-averaged leakage vortex without injection. This is the situation, at which, the interface between the leakage and oncoming flows tends to become parallel to the leading-edge plane. The effect of injection on the boundary layer separation from the casing wall is also investigated. The rotor operation at in-stall condition for both the injection and non-injection cases is studied and the path into instability is discussed for each case. The propagation of a low-velocity region near the pressure surface and leading-edge of the blade was found to be responsible for the detachment of the passage shock from the leading-edge and upstream movement of the leakage vortex, leading to the occurrence of the leading-edge vortex spillage, for both the injection and non-injection cases.

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